linux-kernel - Re: [PATCH v5 3/9] KVM: arm64: Relax locking for kvm_test_age_gfn and kvm_age

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date: Mon, 10 Jun 2024 22:57:54 -0700
From: Oliver Upton <oliver.upton@...ux.dev>
To: James Houghton <jthoughton@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Ankit Agrawal <ankita@...dia.com>,
	Axel Rasmussen <axelrasmussen@...gle.com>,
	Catalin Marinas <catalin.marinas@....com>,
	David Matlack <dmatlack@...gle.com>,
	David Rientjes <rientjes@...gle.com>,
	James Morse <james.morse@....com>, Jonathan Corbet <corbet@....net>,
	Marc Zyngier <maz@...nel.org>,
	Raghavendra Rao Ananta <rananta@...gle.com>,
	Ryan Roberts <ryan.roberts@....com>,
	Sean Christopherson <seanjc@...gle.com>,
	Shaoqin Huang <shahuang@...hat.com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Wei Xu <weixugc@...gle.com>, Will Deacon <will@...nel.org>,
	Yu Zhao <yuzhao@...gle.com>, Zenghui Yu <yuzenghui@...wei.com>,
	kvmarm@...ts.linux.dev, kvm@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v5 3/9] KVM: arm64: Relax locking for kvm_test_age_gfn
 and kvm_age_gfn

On Tue, Jun 11, 2024 at 12:21:39AM +0000, James Houghton wrote:
> Replace the MMU write locks (taken in the memslot iteration loop) for
> read locks.
> 
> Grabbing the read lock instead of the write lock is safe because the
> only requirement we have is that the stage-2 page tables do not get
> deallocated while we are walking them. The stage2_age_walker() callback
> is safe to race with itself; update the comment to reflect the
> synchronization change.
> 
> Signed-off-by: James Houghton <jthoughton@...gle.com>
> ---
>  arch/arm64/kvm/Kconfig       |  1 +
>  arch/arm64/kvm/hyp/pgtable.c | 15 +++++++++------
>  arch/arm64/kvm/mmu.c         | 26 ++++++++++++++++++++------
>  3 files changed, 30 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/arm64/kvm/Kconfig b/arch/arm64/kvm/Kconfig
> index 58f09370d17e..7a1af8141c0e 100644
> --- a/arch/arm64/kvm/Kconfig
> +++ b/arch/arm64/kvm/Kconfig
> @@ -22,6 +22,7 @@ menuconfig KVM
>  	select KVM_COMMON
>  	select KVM_GENERIC_HARDWARE_ENABLING
>  	select KVM_GENERIC_MMU_NOTIFIER
> +	select KVM_MMU_NOTIFIER_YOUNG_LOCKLESS
>  	select HAVE_KVM_CPU_RELAX_INTERCEPT
>  	select KVM_MMIO
>  	select KVM_GENERIC_DIRTYLOG_READ_PROTECT
> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> index 9e2bbee77491..b1b0f7148cff 100644
> --- a/arch/arm64/kvm/hyp/pgtable.c
> +++ b/arch/arm64/kvm/hyp/pgtable.c
> @@ -1319,10 +1319,10 @@ static int stage2_age_walker(const struct kvm_pgtable_visit_ctx *ctx,
>  	data->young = true;
>  
>  	/*
> -	 * stage2_age_walker() is always called while holding the MMU lock for
> -	 * write, so this will always succeed. Nonetheless, this deliberately
> -	 * follows the race detection pattern of the other stage-2 walkers in
> -	 * case the locking mechanics of the MMU notifiers is ever changed.
> +	 * This walk may not be exclusive; the PTE is permitted to change

s/may not/is not/

> +	 * from under us. If there is a race to update this PTE, then the
> +	 * GFN is most likely young, so failing to clear the AF is likely
> +	 * to be inconsequential.
>  	 */
>  	if (data->mkold && !stage2_try_set_pte(ctx, new))
>  		return -EAGAIN;
> @@ -1345,10 +1345,13 @@ bool kvm_pgtable_stage2_test_clear_young(struct kvm_pgtable *pgt, u64 addr,
>  	struct kvm_pgtable_walker walker = {
>  		.cb		= stage2_age_walker,
>  		.arg		= &data,
> -		.flags		= KVM_PGTABLE_WALK_LEAF,
> +		.flags		= KVM_PGTABLE_WALK_LEAF |
> +				  KVM_PGTABLE_WALK_SHARED,
>  	};
> +	int r;
>  
> -	WARN_ON(kvm_pgtable_walk(pgt, addr, size, &walker));
> +	r = kvm_pgtable_walk(pgt, addr, size, &walker);
> +	WARN_ON(r && r != -EAGAIN);

I could've been more explicit last time around, could you please tone
this down to WARN_ON_ONCE() as well?

>  	return data.young;
>  }
>  
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 8bcab0cc3fe9..a62c27a347ed 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -1773,25 +1773,39 @@ bool kvm_unmap_gfn_range(struct kvm *kvm, struct kvm_gfn_range *range)
>  bool kvm_age_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
>  {
>  	u64 size = (range->end - range->start) << PAGE_SHIFT;
> +	bool young = false;
> +
> +	read_lock(&kvm->mmu_lock);
>  
>  	if (!kvm->arch.mmu.pgt)
>  		return false;

I'm guessing you meant to have 'goto out' here, since this early return
fails to drop the mmu_lock.

-- 
Thanks,
Oliver